I'm suffering from the issue of lots of fake registrations for my site.
I'd like to add a captcha for registrations. What is the best plugin that
will require a captcha at registration? Recommendations based on real installs are most appriciated!

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8 Answers
8

My favorite captcha plugin is reCaptcha. The captcha images are scans from old, damaged books. A computer identifies possibilities with OCR, then the user, in correctly filling out the captcha, confirms it. It helps digitize whole books this way.

keycaptcha.com site has live online demo and it serves not only WordPress/BuddyPress powered websites but all PHP5-based CMSs. It is specially designed that in case of being cracked, pool of captchas and even its type (of interactivity for passing) can be changed without plugin reinstall

Other unique features worth to mention:

Customizable language support. It supports a dozen of languages (including German) and requests for new ones are welcome

I rejected edit by Pwn of this answer since I don't feel answer edits are appropriate discussion technique (comments are). But he had (seemingly) valid point that plugins in this answer mostly relate to comments spam, while original question is about registration spam.
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Rarst♦Mar 25 '11 at 20:03

I've used both and haven't had any spam sign ups yet! ... as further security you could install ban-hammer and enable stopforumspam feature that auto-blocks any e-mails on the blacklist from registering!! (do a google search for wordpress banhammer plugin... i'd link you but it won't let me post more than 2 links)